This invention relates to a magnetic resonace apparatus and more particularly to a magnetic resonance apparatus having a radio frequency shield.
FIG. 1 illustrates, in schematic sectional side view, the conventional magnetic resonance apparatus disclosed in The Sixth Conference (August of 1987) of "Society of Magnetic Resonace in Medicine". The conventional magnetic resonance apparatus comprises a superconducting magnet 1 for generating a stationary magnetic field therein,a saddle-shaped radio frequency coil 2 disposed within the magnet 1 for transmitting radio frequency pulses and a receiving magnetic resonance signal and a magnetic field gradient coil 3 disposed between the magnet 1 and the radio frequency coil 2 for generating gradient magnetic field pulses. It is seen that a specimen 4 which is a human body in the illustrated embodiment is inserted into the radio frequency coil 2.
The conventional magnetic resonsance apparatus further comprises an impedance matching circuit connected to the radio frequency coil 2, a receiving pre-amplifier 6 connected to the impedance matching circuit 5 and an input-output terminal 7 connected to the receiving pre-amplifier 6. The magnetic field gradient coil 3 has connected thereto cables 8 to which input-output terminals 9 are connected. The magnetic resonance apparatus also comprises a radio frequency shield member 10 disposed between the radio frequency coil 2 and the magnetic field gradient coil 3. The radio frequency shield member 10 is made of a single continuous electrically conductive sheet such as a copper foil having a thickness of from 35 .mu.m to 70 .mu.m, for example.
During the operation of the conventional magnetic resonance apparatus as described above and shown in FIG. 1, the radio frequency coil 2 which is impedance-matched by the impedance matching circuit 5 generates a radio frequency pulse signal having a frequency band of the order of from 10 MHz to 100 MHz depending upon the signal supplied from the input-output terminal 7 to provide it to the specimen 4.
On the other hand, the magnetic field gradient coil 3 applies gradient magnetic field pulses having a frequency band of the order of from 1 kHz to 10 kHz in accordance with the signal from the input-output terminals 9.
The radio frequency coil 2 then receives the nuclear magnetic frequency signal (NMR signal) from the excited specimen 4 to input through the receiving pre-amplifier 6 and the input-output terminal 7 into an unillustrated computer, wherein the signal is processed by Fourier conversion to form an image of a desired section of the specimen 4.
In order to prevent the radio frequency coil 2 and the magnetic field radient coil 3 from being magnetically coupled to each other and the sensitivity of the radio frequency coil 2 being degraded, the continuous radio frequency shield member 10 is disposed between the radio frequency coil 2 and the magnetic field gradient coil 3.
With the arrangement of the conventional magnetic resonance apparatus as above described, the magnetic coupling beteen the radio frequency coil 2 and the magnetic field gradient coil 3 is prevented by the radio frequency shield member 10 which is a single piece of an electrically conductive sheet. Therefore, an eddy current can be generated in the one-piece radio frequency shield member 10 even by the low-frequency gradient magnetic field pulse signal, causing the waveform of the magnetic field gradient pulse signal to be destorted and resulting in a degraded accuracy of the image of the section of the specimen.